Method for producing foams made of polymers or polymer mixtures and moulded articles made from said foams

ABSTRACT

A method using a two part process line is proposed for manufacturing foams from polymers or polymer mixtures useful for making molded bodies. The process line is divided into at least one first part and one second part. The polymer or polymer mixture is melted in the first part and a gas at a pressure increased relative to the melt pressure is added with the influence of a shearing and/or kneading device on the melted polymer or polymer mixture enriched with gas. In the second part of the process line, foam is formed from the gas-enriched polymer or polymer mixture at a higher pressure than the pressure in the first part of the process line. Membranes can be made from the molded bodies made by the method described above.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for manufacturing foams, especiallymicrocellular foams, from polymers or polymer mixtures for shapingmolded bodies whereby the polymers or polymer mixtures are melted on theassembly line before they are passed through a shaping device in anextruder device and foam formation takes place by introducing a gas, aswell as molded bodies manufacturable by means of the method.

2. Discussion of Related Art

Such a method by which the widest variety of molded bodies can be madeby melting extrusion with the aid of a foam-forming gas have been knownfor a long time for a very wide variety of applications. The previousmethods however are regularly characterized by the fact that the energyexpenditure to work the method was and is very great. For example, amethod is known from DE-OS 44 37 800 in which the pressure load in theextrusion device therein is constant over the temperature that isdifferent in two zones so that an impregnation of the melt with afoam-forming gas is not possible.

From DE-OS 44 37 860, a method is known for manufacturing sheets ofweb-microcellular foams from amorphous thermoplastic plastic byimpregnation of a plastic melt with a volatile propellant in which, in afirst extrusion zone, the thermoplast is impregnated at a temperatureabove its glass temperature with a propellant and in which, in a secondextrusion zone, the propellant-containing melt is cooled to atemperature that is above the glass temperature of thepropellant-containing thermoplast. The propellant-containing melt isthereby cooled by at least 40° C. to a temperature that is at least 30°C. above the glass temperature of the propellant-containing thermoplast.

In other methods for manufacturing foams from polymers, so-callednucleating agents, in the form of talcum or zeoliths for example, or anadditional gas, are necessary.

SUMMARY OF THE INVENTION

Overall, the previous methods are not capable either because of theirregularly high energy consumption and/or possible additional means forinitiating the formation of foam and are not able to producesatisfactory results relative to the products made by the method, sothat it is the goal of the present invention to provide a method of thetype described at the outset with which molded bodies can be madecontinuously with an energy requirement that is very low and free ofadditional substances or materials, i.e., in a continuous processwhereby the method is intended to be able to produce both open-celledand closed molded bodies and with which an adjustment of the pore sizeis possible with considerable and desired accuracy and whereby themethod is simple and can be performed economically by comparison withthe known methods.

This goal is achieved according to the invention by the fact that theprocess line is divided into at least a first part and a second partwhereby in the first part, the melt of the polymer or polymer mixtureand the addition of the gas with pressure greater than the melt takesplace under the influence of a shearing and/or kneading and/orhomogenization means on the polymer or polymer mixture charged with gasand whereby, at the end of the second part of the process line followingthe first part, foam formation of the gas-charged polymer or polymermixture having a higher pressure than the pressure in the first parttakes place.

The advantage of the method according to the invention consistsessentially in the fact that the segmentation chosen according to theinvention in lower pressure and higher temperature-influenced zones andin higher pressure and lower temperature-influenced zones makes possiblethe high solubility of the gas. This therefore permits optimum foamformation whereby the amount of gas in the melt which can be adjusted involume according to the desired property of molded bodies manufacturedaccording to the method, makes possible by the resultant reduction inthe viscosity of the melt, a reduction of the torque to be used, andtherefore a reduction of the energy consumption for working the methodor a device with which the method can be performed.

The advantage of the method according to the invention also lies in thefact that, by dividing or segmenting the process line, the process offoam formation can be influenced under control, while, before thepressure of the melt is increased, the polymer or polymer mixture ismelted and the gas can be introduced and after the pressure is increasedin the area between the pressure-increasing means and the shapingdevice, foam formation, especially microcellular foam formation, cantake place under control.

In an advantageous embodiment of the method, the temperature of the gasintroduced into the first part of the process line is greater than theglass or melting temperature of the polymer or of the polymer mixture sothat the gas introduced initially can distribute itself very well in thepolymer or polymer mixture melt. The temperature in this process line isthen advantageously chosen so that immediately after adding the gas, inother words the actual mixing process of the gas into the polymer or thepolymer mixture, further treatment of the gas-charged polymer or polymermixture melt advantageously takes place with respect to theabove-mentioned shearing, kneading, and homogenization process whileretaining the melting temperature of the polymer or polymer mixture.

The pressure of the gas introduced into the polymer or polymer mixturemelt preferably is greater than 150 bars, whereby the solubility of thegas introduced into the polymer or polymer mixture melt is increased.

Preferably, the temperature of the gas-enriched polymer or polymermixture between the second part of the process line and the shapingdevice is reduced relative to the temperature of the gas-enrichedpolymer or polymer mixture in the first part of the process line,whereby it is also advantageous to increase the pressure of thegas-enriched polymer or polymer mixture between the second part and theshaping device relative to the pressure of the gas-enriched polymer orpolymer mixture in the first part of the process line. The pressureincrease at the end of the first part of the process line bypressure-increasing means, for example in the form of a gear melt pump,makes it possible to reduce the temperature of the gas-charged polymeror polymer mixture melt so that a change in viscosity can be effected ina wide range.

Advantageously, the pressure of the gas-enriched polymer or polymermixture between the second part of the process line and the shapingdevice is in the range of up to 1500 bars, and preferably at least 500bars. Experiments have shown that such pressures are sufficient for acompletely homogeneous dissolution of the gas in the polymer or polymermixture melt, whereby preferably the temperature in this area is up to150° C. below the temperature in the first part of the process line.

The adjustment of pressure in the first and/or second part of theprocess line is preferably adjusted as a function of the type of polymeror polymer mixture and/or a desired pore structure (pore size, openpores, closed pores), whereby all amorphous thermoplastic and partiallycrystalline polymers, copolymers, and polymer blends such aspolycarbonate, polysulfone, polyethersulfone, polypropylene,polyethylene, polyamide, polyester, PVDF, etc., can be used as thepolymers and whereby the polymer mixtures can be mixtures of thepolymers mentioned above, for example.

In addition to adjusting the pressure, it is likewise advantageous toadjust the temperature of the polymer or the polymer mixture, i.e., itsmelts and/or gas-enriched melts, in the first and/or second part of theprocess line, possibly as a function of the type of polymer or of thepolymer mixture and/or a desired pore structure, in order to obtain anoptimum result for the respective polymer and/or polymer mixture and/orthe resultant product with respect to the type of pores (closed, open)and the size of the pores, which simultaneously can serve to reduceenergy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

A molded body (product) manufactured by the method according to theinvention is preferably a membrane, as is used in many methods ofseparating media mixtures (liquid, gaseous). Preferably, the moldedbodies can be formed by the method according to the invention in theform of a flat membrane or also advantageously in the form of a hollowfiber membrane.

The invention will now be described with reference to the attachedschematic drawings with reference to an embodiment.

FIG. 1 shows the schematic design of a device by which the methodaccording to the invention can be worked for making molded bodies, forexample in the form of flat membranes, hollow fiber membranes, or formaking other shaped bodies.

FIG. 2 shows the closed pore structure of a molded body made by themethod according to the invention.

FIG. 3 shows the open pore structure of a molded body made by the methodaccording to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The method according to the invention can be worked with a device 10shown in FIG. 1. For this reason, device 10 will be described first.

Device 10 essentially comprises an extruder device 11 formed by anelongate extruder body in which shearing/kneading/homogenization devices16 are located in a known fashion, for example in the manner of afamiliar screw conveyor, whereby means in the present case in the formof a polymer or polymer mixture 13, which are added in a funnel-shapedinlet 110 to extruder device 11 are conveyed within the extruder device11 to an outlet 111 which is opposed to the inlet 110. The device 10 hasa drive motor 19 and possibly a gear 20 by which the shearing/kneadingdevice or shearing/kneading devices 16 are rotatably coupled with thedrive motor 19. Gear 20, for example, can be a planetary gear.

Temperature control devices 18 are located around the elongate body ofextruder device 11, and can be cooling and/or heating devices.

In a middle part of extruder device 11 is located an opening to supplygas 14 to extruder device 11.

Directly at the outlet 111 of extruder device 11, there is a pump 17 inthe form of a gear pump. The extruder device 11, together with pump 17,forms a first part 150 of the process line 15 in the method according tothe invention.

Following pump 17 a second part 151 of process line 15 is formed with ahead 22, on whose outlet-side end a shaping device 12 is located, withwhich molded bodies in the form of flat membranes, hollow fibermembranes, and other desired shaped bodies can be made, for example. Theexit of the shaped body/product 21 from device 10 at the end of thedevice of the method according to the invention is symbolized by arrow21. In addition, head 22 can have temperature control devices 18 withwhich the gas-enriched polymer or polymer mixture 13 enriched with gasinside head 22 can be subjected to suitable temperature control.

The method according to the invention then operates as follows using thedevice 10 described above.

A polymer or polymer mixture 13, which is for example in the form of agranulate, is added through funnel-shaped inlet 110, the extruder device11 belonging to the first part 150 of process line 15, whereby in thispart 150 the polymer or polymer mixture 13 is initially melted. Themelting temperature is, for example in the case of a polymer such aspolycarbonate, 290° C. Driven by drive motor 19, the molten polymer orpolymer mixture is conveyed by the screw conveyor in the form ofshearing/kneading/homogenizing device 16 for example, into the zone ofextruder device 11, into which gas 14 is introduced at high pressure,for example 150 bars.

The rotating shearing/kneading/homogenization device 16 prevents the gasfrom settling above the melt of the polymer or polymer mixture 13.Instead, gas 14, which can be CO₂ for example, is mixed optimally intothe melt by the rotating shearing/kneading/homogenization device 16 sothat nearly complete dissolution of the gas in the melt takes place. Thetemperature of the gas-enriched melt in the extruder device 11 islowered relative to the original melting temperature by the additionaltemperature control means 18 located after the position of gasintroduction at the extruder device 11 and kept essentially constant tothe outlet 111, whereby the lowering of the temperature, for examplerelative to the melt temperature of 290° C. to 225° C., takes place in apolymer formed by polycarbonate. In the last section of the first part150 of the process line, by means of pump 17 (gear melt pump), whichforms a pressure barrier for the following second part 151 of theprocess line 15, there is a considerable increase of the gas-enrichedpolymer or polymer mixture melt pressure, i.e., in head 22 which is madetubular. In head 22, because of the pressure increase, there is anotherincrease in the dissolution of the gas in the polymer or in the polymermixture melt. The result is that the temperature control devices 18,located in the second part 151 of the process line 15, i.e., in thetubular head 22, can be used to reduce the temperature of thegas-enriched melt made of polymer or polymer mixture 13.

As a result of the activity of pump 17, the gas-enriched polymer orpolymer mixture melt 13, raised to 1500 bars or more in head 22, ispushed out of the shaping device 12 so that at least after thegas-enriched polymer or polymer mixture melt 13 emerges from the shapingdevice 12, the polymer or polymer mixture foam emerges, for example as aflat membrane or hollow fiber membrane, following suitable shaping byshaping device 12.

All the pressures and temperatures of the polymer or the polymer gasmixture 13, including the gas 14, are suitably adjustable to the polymeror the polymer mixture 13 used and optimizable by means of theabove-mentioned devices used, such as pump 17,shearing/kneading/homogenizing (mixing) device 16, and temperaturecontrol means 18 (cooling, heating).

EXAMPLE

In a uniform double-screw extruder with a geometry of 25 mm insidediameter and an L/D ratio of 32 D, a polycarbonate granulate was meltedat 290° C. and subjected under a pressure of 140 bars with 8 wt. % CO₂(FIG. 1). The polycarbonate used has a melted volume index MVI (300°C./1.2 kg) of 9.5 ml/10 min. In the five extruder segments following thegas injection point, the melt temperature was lowered corresponding tothe dissolved volume of gas in the polymer to 225° C. The pressure isincreased in the connected gear melt pump to about 650 bars. Under thesepressure conditions, the solubility of the gas is increased further sothat the melting temperature in this segment can be reduced to 170° C.At the outlet of an annular hollow fiber nozzle, an open-celledpolycarbonate foam appears with an average cell size of 10 μm and a celldensity of 2×10⁹ cells/cm³ (FIG. 2). The permeability of N₂ is 1.2m³/m²·h·bar, that of O₂ is 1.1 m³/m²·h·bar, and of He is 2.2m³/m²·h·bar.

When the melt pressure is 320 bars instead of 650 bars as mentionedabove in this segment under the above-described experimental conditionswith the same experimental arrangement, there occurs at the outlet of anannular hollow fiber nozzle a polycarbonate foam with a predominantlyclosed structure. The average cell size in this hollow fiber is then 13μm and the cell density is 7×10⁸ cells/cm³ (FIG. 3).

LIST OF REFERENCE NUMERALS

10 Device

11 Extruder device

110 Inlet

111 Outlet

12 Shaping device

13 Polymer/polymer mixture

14 Gas

15 Process line

150 First part

151 Second part

16 Shearing/kneading/homogenization device

17 Pump

18 Temperature control device (cooling, heating)

19 Drive motor

20 Gear

21 Shaped body/product

22 Head

What is claimed is:
 1. A method for manufacturing foam from a polymer ora polymer mixture, comprising: melting the polymer or the polymermixture in an extruding device of a first part of a process line under afirst pressure; enriching the melted polymer or the melted polymermixture with a gas at a gas pressure in the first part of the processline while influencing the melted polymer or the melted polymer mixturewith at least one shearing, mixing, or kneading device, wherein the gaspressure is higher than the first pressure; and forming the foam fromthe gas enriched melted polymer or melted polymer mixture at the end ofa second part of the process line following the first part while the gasenriched polymer or polymer mixture at the end of the second part of theprocess line has a second pressure that is higher than the firstpressure.
 2. The method according to claim 1, wherein a temperature ofthe gas for enriching the melted polymer or the melted polymer mixtureis higher than a glass or melting temperature of the polymer or thepolymer mixture.
 3. The method according to claim 1, wherein the processof enriching the polymer or the polymer mixture with the gas occurswhile maintaining a melting temperature of the polymer or polymermixture.
 4. The method according to claim 1, wherein the gas pressure ofthe gas is more than 150 bars.
 5. The method according to claim 1,wherein a temperature of the gas-enriched melted polymer or the meltedpolymer mixture between the second part of the process line and ashaping device is reduced with respect to a temperature of thegas-enriched melted polymer or the melted polymer mixture in the firstpart of the process line.
 6. The method according to claim 1, whereinthe increase in pressure from the first part of the process line to thesecond part of the process line is effected at the end of the firstprocess line with a gear pump.
 7. The method according to claim 1,wherein the second pressure is about 1500 bars or less.
 8. The methodaccording to claim 1, wherein the first pressure, second pressure andgas pressure are adjusted as a function of a composition of the polymeror the polymer mixture, a desired pore structure, or both.
 9. The methodaccording to claim 1, wherein a temperature of the gas and a temperatureof the gas-enriched melted polymer or melted polymer mixture in thefirst part of the process line or second part of the process line orboth parts of the process line is adjusted as a function of acomposition of the polymer or the polymer mixture, a desired porestructure, or both.
 10. The method according to claim 1, wherein at theend of the second part of the process line, the foam is shaped into amolded body in a shaping device.
 11. The method according to claim 10,wherein the molded body is a membrane.
 12. The membrane according toclaim 11, wherein the membrane has a microcellular structure of 0.5 to15 μm and a wall thickness of 0.025 to 100 mm.
 13. The molded bodyaccording to claim 10, wherein the molded body is a flat membrane. 14.The molded body according to claim 10, wherein the molded body is ahollow fiber membrane.
 15. The method according to claim 1, wherein thefoam is a microcellular foam.